Digital microfluidics generally refers to microfluidic technology using manipulation of fluid droplets. Droplets of unit size may be controlled, combined, reacted, analyzed, and/or stored. The fluid droplets may be moved by applying electric fields in proximity to the droplets. Accordingly, digital microfluidic devices may include a substrate patterned with electrodes and coated with a dielectric insulator and a hydrophobic film. By applying an electric field in an area adjacent a droplet, interfacial (e.g. electrowetting) and body forces are generated, which may attract the droplet into the region of higher field intensity, thus moving the droplet.
Generally, so-called ‘open format’ and ‘closed format’ digital microfluidic devices are available. FIGS. 1A-1B illustrate portions of an open and closed format digital microfluidic device, respectively.
As shown in FIG. 1A, an open format digital microfluidic device may include grounded traces, such as electrode 105 patterned on an electrode substrate 115. A droplet, such as the droplet 125 may be added to the device by using a pipette or eyedropper to place a desired amount of fluid on the device. A voltage may be applied, for example to another electrode 110, which may alter the contact angle between the droplet and substrate 115 and/or produce a localized electric field gradient, resulting in motion of the droplet 125 in the direction indicated by arrow 127 (e.g. toward the electrode 110). The droplet 125 will generally move until it is situated between the two electrodes shown. Typically, in open format devices, there may be a continuous ground electrode (e.g. a trace) running parallel to the stepping stone-like path (indicated schematically in FIG. 1A by a ground symbol shown above the droplet).
As shown in FIG. 1B, a closed format digital microfluidic device may include two substrates 150 and 155. A droplet, such as the droplet 164, may be placed on the substrate 150 prior to placing the lid 155 over the droplet 164. A spacer element, such as double-sided adhesive tape, a gasket, or a patterned layer, may be used to achieve a gap between the substrates 150 and 155 of about 10-1000 microns, bounding the placed droplet 164 on upper and lower surfaces. In some system, holes may be provided in the substrates 150 or 155 to provide reservoirs for fluid addition to the system. The substrate 155 (e.g. the lid) may include a ground plane electrode 160, while the substrate 150 may include an actuated electrode 162. When a voltage is applied to the electrode 162, an electric field may be generated between the electrode 162 and the electrode 160, exerting body forces on the droplet and altering the contact angle between the droplet and the surface of the substrate 150, 155, or both, and the droplet 164 may move toward the electrode 162 as shown in FIG. 1B. The space between the substrates 150 and 155 may be filled with immiscible fluid through which the droplet 164 may be moved, or may contain a gas or vacuum. During operation, under static conditions the digital microfluidic electrodes may be maintained in a grounded state. When droplet actuation (e.g. movement) is desired, an electrode nearest the droplet in the direction of desired motion may be activated (e.g. energized, a voltage being applied to the electrode). The resultant electric field may cause the droplet to move stepwise onto the energized pad as a result of electrowetting and/or body forces. To remove droplets, the lid is generally removed and the droplet aspirated.